Conventionally, insulated wires in which conductors are covered with covering materials made from a resin composition are frequently used in carrying out wiring of parts for a car and electric/electronic equipment. In this type of insulated wires, especially in the insulated wire to be used in a section where heat resistance is required, used as the covering material is a vinyl chloride resin composition of which heat resistance is improved by adjusting types, amounts or polymerization degrees of a thermal stabilizer and a plasticizer to be added thereto, or by forming cross-links therein.
However, there is a problem that this type of vinyl chloride resin composition includes halogen elements, so that it emits harmful halogenous gas into the atmosphere in case of car fire or at the time of combustion for disposing of electric/electronic equipment by incineration, causing environmental pollution.
Therefore, from the view point of reducing loads on the global environment, the vinyl chloride resin composition has been recently replaced with a so-called non-halogenous flame-retardant resin composition, which is prepared by adding a metallic hydrate such as magnesium hydroxide as a non-halogenous flame retardant to an olefin resin such as polyethylene.
However, the olefin resin is essentially combustible, and the non-halogenous flame retardant is inferior to a halogenous flame retardant in effect of flame retardancy. For these reasons, the non-halogenous flame-retardant resin composition requires a large amount of metallic hydrate to be added thereto in order to secure sufficient flame retardancy, thus bringing a disadvantage that mechanical properties such as wear resistance, tensile elongation and tensile strength remarkably degrade.
In order to overcome such a disadvantage, Japanese Patent Gazette No. 3280105, for example, discloses a flame-retardant resin composition which is prepared by adding a metallic hydrate and a cross-linking auxiliary agent to a resin ingredient containing polyethylene or an alpha-olefin copolymer and an ethylene copolymer or a rubber, and by making the resin ingredient further contain a specific functional group.